U.S. patent number 7,885,351 [Application Number 11/760,175] was granted by the patent office on 2011-02-08 for method of correcting gain and phase imbalance of a multi-carrier transmission signal, transmitter and base station.
This patent grant is currently assigned to Alcatel Lucent. Invention is credited to Thomas Bitzer, Thomas Bohn, Andreas Pascht, Jens Strauss.
United States Patent |
7,885,351 |
Bitzer , et al. |
February 8, 2011 |
Method of correcting gain and phase imbalance of a multi-carrier
transmission signal, transmitter and base station
Abstract
Method of correcting a gain and phase imbalance of an analogue
modulator (32) for multiple channels (CHi) of a multi-carrier
transmission signal, the method comprising the steps of determining
a gain imbalance correction factor (GCFi) and phase imbalance
correction factor (PCFi) for each channel individually and applying
said correction factors (GCFi, PCFi) to the corresponding channel
(CHi) individually, before the multi-carrier synthesis of the
channels is done, whereas step a) for each one of the multiple
channels (CHi) is performed in a time-multiplexed manner with step
a) for the other ones of the multiple channels (CHi).
Inventors: |
Bitzer; Thomas (Schorndorf,
DE), Pascht; Andreas (Schorndorf, DE),
Bohn; Thomas (Stuttgart, DE), Strauss; Jens
(Althengstett, DE) |
Assignee: |
Alcatel Lucent (Paris,
FR)
|
Family
ID: |
37269482 |
Appl.
No.: |
11/760,175 |
Filed: |
June 8, 2007 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20080049866 A1 |
Feb 28, 2008 |
|
Foreign Application Priority Data
|
|
|
|
|
Aug 28, 2006 [EP] |
|
|
06300897 |
|
Current U.S.
Class: |
375/296;
375/260 |
Current CPC
Class: |
H04L
27/364 (20130101); H04L 27/2626 (20130101) |
Current International
Class: |
H04L
25/49 (20060101) |
Field of
Search: |
;375/260,296 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
WO 2004/082232 |
|
Sep 2004 |
|
WO |
|
Other References
Kirkland W R, et al.: "I/Q distortion correction for OFDM direct
conversion receiver," Electronics Letters, IEE Stevenage, GB, vol.
39, No. 1, Jan. 9, 2003, pp. 131-133, XP006019480. cited by
other.
|
Primary Examiner: Liu; Schuwang
Assistant Examiner: Huang; David
Attorney, Agent or Firm: Fay Sharpe LLP
Claims
The invention claimed is:
1. A method of correcting a gain and phase imbalance of an analogue
modulator for multiple digital channels of a multi-carrier
transmission signal, wherein the different channels are to be
transmitted on different carrier frequencies, the method comprising
the steps of a) determining a gain imbalance correction factor and
phase imbalance correction factor for each channel individually and
b) applying said correction factors to the corresponding channel
individually, before a multi-carrier synthesis of the channels is
done, wherein step a) for each one of the multiple channels is
performed in a time-multiplexed manner with step a) for the other
ones of the multiple channels, whereby said correction factors
being applied in a continuous manner, using new correction factors
as soon as new correction factors have been determined; and wherein
step a) further comprises the steps of: ai) channel-separating a RF
signal output by said modulator, aii) comparing the
channel-separated modulator output signal with the corresponding
input channel signal for each channel individually.
2. The method according to claim 1, wherein aL,H) said step a) is
performed first for the channels corresponding to the lowest-band
and highest-band carriers by comparing the channel-separated
modulator output signal with the corresponding input channel signal
of the lowest-band carrier and of the highest-band carrier and said
step a) is then performed for the other ones of the multiple
channels by linear approximation from said gain and phase imbalance
correction factors determined in step aL,H).
3. The method according to claim 1, wherein said channel separation
step comprises the steps of digitizing the RF signal output by said
modulator separating the channels in said digitized version of the
RF signal output by said modulator.
4. The method according to claim 1, wherein said channel separation
step comprises the steps of separating the channels in the analogue
RF signal output by said modulator to obtain an analogue signal for
each channel digitizing said analogue signal for each channel
individually.
5. A transmitter for performing the method of claim 1, comprising a
digital part and an analogue part, the digital part comprising a
multi-carrier synthesis module, a determination module for
determining gain and phase imbalance at one channel and a
correction module for each channel for applying gain and phase
correction factors determined by said determination module to each
corresponding channel individually, whereby the correction factors
are applied to the respective correction modules, using new
correction factors as soon as new correction factors have been
determined, the analogue part comprising an analogue modulator, the
transmitter further comprising a feedback path from an output of
said analogue modulator to said determination module, wherein said
determination module channel-seperates an RF signal output by said
modulator and compares the channel-seperated modulator output
signal with the corresponding input channel signal for each channel
individually.
6. A base station comprising the transmitter of claim 5.
Description
The invention is based on a priority application EP 06 300 897.3
which is hereby incorporated by reference.
TECHNICAL FIELD
The present invention relates to the suppression of unwanted image
signals for broadband signals in multi-carrier operation.
BACKGROUND OF THE INVENTION
Future base station transmitters will have to process an increasing
number of carriers or sub-carriers, e.g. for GSM (Global System for
Mobile communication), UMTS (Universal Mobile Telecommunication
System) or WiMAX (Worldwide Interoperability for Microwave Access).
This leads to an increasing signal bandwidth to be handled. A
multi-carrier transmitter comprises an input for receiving digital
signals on multiple different channels. The signals on the
different channels are to be transmitted on different carrier
frequencies. For modulating the signals to their corresponding
carrier frequencies, a modulation technique called IQ modulation is
known. For IQ modulation the digital signal to be modulated is
split into a in-phase (I) component and a quadrature (Q) component.
Using an IQ modulator the I and Q components of the signal are then
modulated on the carrier frequency with a phase shift of
90.degree..
There also is a trend towards direct-up conversion in the
transmitter. Using direct-up conversion, the different channels are
directly converted to their corresponding carrier frequencies
without the use of an intermediate frequency (IF). Up-conversion is
usually done using an analogue modulator, often an IQ modulator.
The direct-up architecture has a number of advantages compared to
solutions with intermediate frequencies. For example, there are
less components needed and the performance of DA-converters
(digital-to-analogue converters) is best near DC (direct
current).
Analogue modulators generate unwanted image signals in the
multi-carrier signals due to imbalances. In case of IQ modulators
the imbalances comprise gain differences in between the I path and
the Q path and a phase deviation between the I path and the Q path,
which is not exactly 90.degree.. In the case of direct-up
conversion, the unwanted images fall into the signal band itself
and in general cannot be eliminated by filtering. Furthermore, said
imbalances depend on the frequency of the signal, which makes image
compensation for multi-carrier signals covering a wide frequency
range difficult.
According to standard requirements, all transmitted signals have to
fulfill certain spectral masks and the images have to be limited
below certain levels. High requirements are for example given in
the GSM 900 standard for GSM transmission in the 900 MHz band. Good
image suppression is therefore needed for the realization of
multi-carrier transmitters for the GSM 900 band.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a method for correcting
the gain and phase imbalance of an analogue modulator for multiple
digital channels of a multi-carrier transmission signal.
It is also an object of the invention to provide a transmitter for
performing said method and a base station comprising said
transmitter.
These objects, and others that appear below, are achieved by a
method of correcting a gain and phase imbalance of an analogue
modulator for multiple digital channels of a multi-carrier
transmission signal, the method comprising the steps of a)
determining a gain imbalance correction factor and phase imbalance
correction factor for each channel individually and b) applying
said correction factors to the corresponding channel individually,
before the multi-carrier synthesis of the channels is done, whereas
step a) for each one of the multiple channels is performed in a
time-multiplexed manner with step a) for the other ones of the
multiple channels, a corresponding transmitter for performing the
method of claim 1, comprising a digital part and an analogue part,
the digital part comprising a multi-carrier synthesis module, a
determination module for determining gain and phase imbalance at
one channel and a correction module for each channel for applying
gain and phase correction factors determined by said determination
module to each corresponding channel individually, the analogue
part comprising an analogue modulator, the transmitter further
comprising a feedback path from an output of said analogue
modulator to said determination module and a corresponding base
station comprising the inventive transmitter of claim 6.
According to the inventive method, the gain and phase imbalances
are determined for each channel individually. The determined
correction factors for the channels are then applied to each
channel individually in the digital part of the transmitter before
the multi-carrier synthesis is done. The determination of the
correction factors for one of the channels is time multiplexed with
the determination of the correction factors for the other ones of
the channels. This leads to an improved image suppression, as the
gain and phase imbalances depend on frequency. Within one channel,
the imbalances are approximately constant, so that a good image
suppression can be achieved. For the other channels which are to be
transmitted on other frequencies, the gain and phase imbalance
factors are determined separately and individually, so that for
these channels good image suppression can be achieved too.
According to a preferred embodiment of the invention, the gain and
phase imbalances are determined for the channels which are to be
transmitted on the carrier at the lowest frequency and on the
carrier at the highest frequency. For the channels which are to be
transmitted on carriers at frequencies in between said lowest and
highest frequency, the correction factors are determined by a
linear approximation of the correction factors for the channels
which are to be transmitted on the carriers at the lowest frequency
and on the carrier at the highest frequency. This leads to a very
precise determination of the imbalances at the frequencies of the
carriers, which in turn enables a very precise reduction of the
images independently of the frequency dependencies of the
imbalances. The correction factors for the channels which are to be
transmitted on the carrier at the lowest frequency and on the
carrier at the highest frequency are determined by evaluating an
output signal of the modulator for the respective channel which is
fed to the determination module over a feedback loop. Said
evaluation is done in a multiplexed mode. The imbalance correction
is done by applying the determined correction factors to the
channels before the multi-carrier synthesis is done.
In another preferred embodiment of the invention the gain and phase
imbalances are determined for all the channels which are to be
transmitted individually. This leads to a very precise
determination of the imbalances at the frequencies of the
individual carriers and in turn enables a very precise reduction of
the images independently of the frequency dependencies of the
imbalances. The correction factors for the channels are determined
by evaluating an output signal of the modulator for the respective
channel which is fed to the determination module over a feedback
loop. Said evaluation is done in a multiplexed mode. The imbalance
correction is done by applying the determined correction factors to
the channels before the multi-carrier synthesis is done.
Said evaluation of the output signal of the modulator for the
respective channel which is fed to the determination module over a
feedback loop for either one of the above mentioned preferred
embodiments can be done by first digitizing the RF signal output by
said modulator and then separating the channels in the digital
domain.
Said evaluation of the output signal of the modulator for the
respective channel which is fed to the determination module over a
feedback loop for either one of the above mentioned preferred
embodiments can also be done by separating the channels in the
analogue RF signal output by said modulator to obtain an analogue
signal for each channel and then digitizing said analogue signal
for each channel individually.
With the inventive method unwanted images of multi-carrier signals
can be reduced much more precisely. It is not necessary to know the
kind of frequency dependency of the imbalances for the operation of
the proposed method. This allows multi-carrier signals to be
distributed in a larger bandwidth. Due to the fact that direct-up
conversion of multi-carrier signals is enabled with a good image
suppression, the available digital-to-analog converters can be
operated with a better performance, allowing for e.g. better
multi-carrier GSM transmitters. The method will also work for UMTS
or WiMAX signals with a huge number of sub-carriers.
Using the proposed method, it is possible to perform channel-wise
imbalance correction using only one feedback path.
BRIEF DESCRIPTION OF THE DRAWINGS
Other characteristics and advantages of the invention will become
apparent in the following detailed description of preferred
embodiments of the invention illustrated by the accompanying
drawings given by way of non-limiting illustrations. The same
reference numerals may be used in different figures of the drawings
to identify the same or similar elements.
FIG. 1 shows a schematic flowchart of steps a) and b) of the method
being performed for each channel CHi,
FIG. 2 shows a schematic overview of an embodiment of an inventive
transmitter,
FIG. 3 shows an illustration of unwanted image signals and their
compensation and
FIG. 4 shows a schematic overview of another embodiment of an
inventive transmitter.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
FIG. 1 shows a schematic flowchart of steps a) and b) of the method
according to the present invention.
For each channel CHi, for i=1 . . . N, of the channels which are to
be transmitted on a multi-carrier signal, in step a) a gain
imbalance correction factor GCFi and phase imbalance correction
factor PCFi is determined individually. Thus for channel CH1, the
correction factors GCF1 and PCF1 are determined, for channel CH2,
the correction factors GCF2 and PCF2 are determined and so on. In
step b) said correction factors GCFi, PCFi are applied to the
corresponding channel CHi individually, before the multi-carrier
synthesis of the channels is done. Step a) for each one of the
multiple channels CHi is performed in a time-multiplexed manner
with step a) for the other ones of the multiple channels CHi.
During a multiplex time window, the correction factors GCFi and
PCFi are determined. The correction factors are applied to the
respective correction modules CORRi preferably in a continuous
manner, using new correction factors GCFi and PCFi as soon as new
correction factors GCFi and PCFi have been determined.
FIG. 2 shows a schematic overview of an embodiment of an inventive
transmitter 10. FIG. 4 shows a schematic overview of another
embodiment of an inventive transmitter 10. The same numerals in
both figures denote the same or similar parts of the transmitter
10.
The structure of the transmitter 10 comprises a digital part 20 and
an analogue part 30. The digital part 20 and the analogue part 30
are connected by digital-to-analogue converters 42, 44 and an
analogue-to-digital converter 46. In the digital part 20 each
channel CHi passes through correction module CORRi, where gain and
phase imbalances are corrected by applying correction factors GCFi
and PCFi. The correction factors GCFi and PCFi will compensate for
the IQ-imbalances of the IQ modulator 32.
The digital part 20 further comprises a multi-carrier synthesis
module 22, where the I and Q data of each channel CHi output from
the respective modules CORRi are input and where the signal data
are first multiplied with different factors in such a manner that
the channels will be shifted to different carrier frequencies in
the analogue domain. After that, all I and Q data are summed which
leads to one common I and one common Q output.
After the digital-to-analogue conversion in module 42 for the I
part and in module 44 for the Q part, the I and Q signals are input
to the IQ modulator 32 which introduces the IQ imbalances that have
to be compensated. In the transmitter 10 the RF output signal of
the IQ modulator 32 is then input to an amplifier chain to increase
the power level of the signal. The amplifier chain is not shown in
FIG. 2.
The feedback path 36 shown in FIG. 2 and FIG. 4 feeds the RF signal
output by the IQ modulator 32 to the detection unit 24. The
feedback path comprises a down-conversion mixer 34, an
analogue-to-digital converter 46 and an IQ demodulator 26. The
digital IQ demodulator 26 generates an I and a Q signal that are
evaluated in the determination module 24. This evaluation is
carried out in a time-multiplexed manner and results in gain and
phase correction factors GCFi, PCFi for each of the channels
CHi.
In FIG. 2 said evaluation is carried out for the low band carrier
and for the high band carrier resulting in one gain and one phase
correction factor for each of the two channels. This means that the
gain and phase imbalance correction factors GCF1, GCFN, PCF1 and
PCFN for the channels CHI and CHN which are to be transmitted on
the carrier at the lowest frequency and on the carrier at the
highest frequency are determined in module 24 from a comparison of
the I and Q data from the feedback path to the input I and Q data
of the channels 1 and N. The gain and phase imbalance correction
factors GCF2 . . . GCFN-1, PCF2 . . . PCFN-1 for the inner channels
CH2 . . . CHN-1 are determined in module 24 by linear approximation
of GCF1 and GCFN and of PCF1 and PCFN. The determined values for
said correction factors are applied to the correction modules CORRi
of the respective channels CHi. It is advantageous that the
multi-carrier synthesis is carried out after a channel-wise gain
and phase imbalance correction. This allows to correct the
different channels with different correction factors. This means
that a frequency dependency of the gain and phase imbalance in the
IQ modulator 32 can be taken into account and be compensated
without having to know the character of the frequency
dependency.
An example of a possible determination of the correction factors
GCF1, GCFN, PCF1 and PCFN for the embodiment of the invention given
in FIG. 2 will in the following be illustrated using FIG. 3. In the
upper half of FIG. 3, the signals of the different channels are
shown together with respective image signals of other channels. The
high-band signal SH together with the image signal IL is filtered
by filter FH. The low-band signal SL together with the image signal
IH is filtered by filter FL. The low-band signal SL including the
image of the high-band signal IH and the high band signal SH
including the image of the low-band signal IL are after filtering
the basis of the estimation of the gain and phase imbalance
values.
In the embodiment of the invention shown in FIG. 4, the digital IQ
demodulator 26 generates an I signal and a Q signal that are
evaluated in the determination module 24. This evaluation is
carried out for each of the channels individually. It is carried
out in a time-multiplexed manner and results in gain and phase
correction factors GCFi, PCFi for each of the channels CHi. Two
alternatives of the processing of the signal in the feedback path
and the following evaluation in the determination module 24 are
imaginable.
In the first alternative the multi-carrier signal could be
processed as a whole in the feedback path if the signal bandwidth
doesn't exceed the limitations given by the analogue parts and the
analogue-to-digital converter. In this alternative the
determination module 24 carries out the channel separation as the
first step. After that it compares the signal in one detected
channel from the feedback path to the original signal of that
channel in a time multiplex manner.
In the second alternative the down-conversion mixer 34 in the
analogue part 30 could be driven with a switched local oscillator
(LO) source that generates different LO frequencies. In this
alternative it would be possible to process only one of the
channels at the same time in the analogue domain and the
analogue-to-digital converter 46. Thus time-multiplex is already
introduced in the analogue part 30 by the down-conversion mixer 34.
This would relax the bandwidth requirement for the AD-converter 46
and would therefore be suitable if the signal bandwidth exceeds the
limit of an available analogue-to-digital converter. On the other
hand, a slightly increased effort in the analogue domain would
result due to the need for a switchable LO source. In this
alternative the determination module 24 compares the detected
signal at its input and the original signal of one channel.
The determination module 24 determines one gain imbalance
correction factor GCFi and one phase imbalance correction factor
PCFi for each of the channels CHi. The determined values for said
correction factors are applied to the correction modules CORRi of
the respective channels CHi. It is advantageous that the
multi-carrier synthesis is carried out after a channel-wise gain
and phase imbalance correction. This allows to correct the
different channels with different correction factors. This means
that a frequency dependency of the gain and phase imbalance in the
IQ modulator 32 can be taken into account and be compensated
without having to know the character of the frequency
dependency.
* * * * *